In Mission 4, after exiting the main building into the Junkyard and you are near going up the ramp, do not go up the ramp. Next to the ramp is a small path that leads to the Junkyard King. He has a lot of HP. It will take some time to defeat him.

Trophies

Successfully complete one of the following tasks to get a trophy:

I have a friend (Bronze): Complete a level in Co-op.

Collector (Bronze): Collect all 8 hidden comic books.

Big Daddy Expert (Bronze): With Big Daddy, use the flashbang to stun 6 enemies at once.